What is the best method, or methods, to use in screening for strabismus and amblyopia? Most studies of the matter have used only a single test modality, and the few multi-modality comparison studies have had important limitations. The present proposal is an epidemiologic case control study comparing several screening test modalities. Carefully optimized versions of the tests used are incorporated, to demonstrate each modality's full potential. The modalities chosen seem most promising for practical screening application: Linear Snellen-equivalent visual acuity, isotropic photorefraction, off-axis photorefraction, and random dot stereogram testing. All four modalities will be tested on each subject in a masked protocol, with a full ophthalmological examination, also will consist of randomly interleaved matched populations of normals, and patients chosen to represent a full spectrum of major to minor strabismic, amblyopic and amblyopiagenic refractive deficits. The study population will be 3 to 4 year-olds, who are: (a) old enough to allow subjective sensory testing and to avoid the increased variability of infant testing; (b) young enough to still fall within the developmental plasticity interval, neurally and anatomically; and who (c) themselves constitute a screening target population. The goal of the study will be to determine which test modality, or which modality combination, is required to detect strabismic, amblyopic, and amblyopiagenic refractive deficits with optimal sensitivity and specificity.